Air is pretty unintrusive when it comes to photography, but what do you do when air is the thing you want to take pictures of? You turn to Schlieren photography.
August Toepler invented the process in 1864 to study supersonic motion, and it soon became a mainstay in aeronautics, where the study of airflow is crucial. The idea is remarkably clever. Collimated light (light in which the rays are parallel and thus, do not disperse with distance), is shined onto the subject. Any airflow will cause density gradients that in turn cause variations in the refractive index (the ratio of the speed of light in a vacuum vs that medium and a measure of how much that medium bends light). These variations then cause the previously collimated light to diverge, and as it spreads apart, it shines with differing intensities on different spots, depending on how spread apart it is.
In this implementation, the light is focused onto a razor blade. Some of the light that is bent by the differences in air density is blocked by the blade, causing the variations to appear as dark and light areas; in other words, the intensity of the light corresponds directly with the density of the air. The diagram below shows a typical setup:
Check out the video above to see some cool shots using the technique!